What is a Capacitor?

Capacitors for primary systems are available in 50- to 300-kvar single phase

A capacitor, also called a condenser, is thus essentially a sandwich of two plates of conducting material separated by an insulating material, or dielectric. Its primary function is to store electrical energy. Capacitors differ in thesize and geometrical arrangement of the plates and in the kind of dielectric material used. Hence, they have such names as mica, paper, ceramic, air, and electrolytic capacitors.

Their capacitance may be fixed or adjustable over a range of values for use in tuning circuits. The energy stored by a capacitor corresponds to the work performed (by a battery, for example) in creating opposite charges on the two plates at the applied voltage.

The amount of charge that can be stored depends on the area of the plates, the spacing between them, the dielectric material in the space, and the applied voltage. A capacitor incorporated in an alternating-current (AC) circuit is alternately charged and discharged each half cycle. The time available for charging or discharging thus depends on the frequency of the current, and if the time required is greater than the length of the half cycle, the polarization (separation of charge) is not complete.

Under such conditions, the dielectric constant appears to be less than that observed in a direct-current circuit and to vary with frequency, becoming lower at higher frequencies. During the alternation of polarity of the plates, the charges must be displaced through the dielectric first in one direction and then in the other, and overcoming the opposition that they encounter leads to a production of heat known as dielectric loss, a characteristic that must be considered when applying capacitors to electrical circuits, such as those in radio and television receivers. Dielectric losses depend on frequency and the dielectric material.

Except for the leakage (usually small) through the dielectric, no current flows through a capacitor when it is subject to a constant voltage. Alternating current will pass readily, however, and is called a displacement current.

Capacitor Application for Power Factor Correction.

It is desirable to add shunt capacitors in the load area to supply the lagging component of current. The cost is frequently justified by the value of circuit and substation capacity released and/or reduction in losses. Installed cost of shunt capacitors is usually least on primary distribution systems and in distribution substations.

Capacitor Installations

Capacitors for primary systems are available in 50- to 300-kvar single phase units suitable for pole mounting in banks of 3 to 12 units. Capacitors should be connected to the system through fuses so that a capacitor failure will not jeopardize system reliability or result in violent case rupture.

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